Dopamine-beta-hydroxylase (DBH), the enzyme that catalyzes the conversion of dihydroxyphenylethylamine to the neurotransmitter norepinephrine, is localized to the catecholamine-containing vesicular structures in the adrenal medulla and in sympathetic nerves. DBH is released in response to stimulation of both the adrenal medulla and adrenergic nerves and is found circulating in the blood. It is proposed that advantage be taken of the unique characteristics of this enzyme to study the following problems: (a) the mechanisms of release of norepinephrine and DBH from sympathetic nerves, (b) the effect of drugs which alter sympathetic nervous system function upon the release of DBH, and (c) the metabolic fate of DBH after it is released into the circulation. The first problem will be investigated by using the unique localization of DBH to catecholamine-containing vesicles to examine any changes in vesicular density which might occur after neural stimulation. Such studies will help to clarify the mechanism of the release process as well as contribute to a better understanding of the fate of the vesicular structures themselves. These experiments will be done with preparations obtained both from peripheral sympathetic nerves and from the brain. The second problem to be investigated will involve the study of the activity of DBH in sympathetically innervated tissues and blood of experimental animals before and after treatment with drugs which affect the sympathetic nervous system. It is anticipated that these studies will help to clarify the mechanisms of action of these drugs. Such an investigation is also of value because of the increasingly frequent measurement of human plasma DBH activity in clinical situations in which such drugs are being used. Finally, it would be of value to have available more information regarding the metabolic fate and half- life of this enzyme in the circulation. These data are particularly necessary because of the increasing use of plasma DBH measurements in clinical settings. Approaches to the study of the problem of the metabolic fate of DBH will include the injection of purified, radioactively labeled enzyme preparation into experimental animals to make it possible to localize the site(s) of metabolism of DBH.